Knight departs during broadcast's lowest point (5)
I believe the answer is:
nadir
'lowest point' is the definition.
(I've seen this before)
'knight departs during broadcast's' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'during' is an insertion indicator.
'broadcast' becomes 'air' (synonyms).
'd' inserted inside 'air' is 'adir'.
'n'+'adir'='NADIR'
(Other definitions for nadir that I've seen before include "Low point" , "Drain into the lowest point (5)" , "pits" , "least successful point" , "Dinar (anag.)" .)